The gallium nitride compound semiconductor is used much as devices such as a light emitting diode and a laser diode. In this production process of a gallium nitride compound semiconductor (gallium nitride compound semiconductor process), a gallium nitride compound is vapor-grown on a substrate of sapphire or the like by the MOCVD method. As the source gas used for the production process, for example, trimethylgallium, trimethylindium, and trimethylaluminum as the group III metal source and ammonia as the group V nitrogen source are used. In addition to these source gases, hydrogen and nitrogen are used as the carrier gases.
Ammonia used as the source gas of the production process is highly-pure ammonia generated by distilling or rectifying ammonia for industrial purposes or high-priced ammonia generated by purifying this highly-pure ammonia because it is required to have an extremely high purity. On the other hand, hydrogen used as a carrier gas is produced industrially, for example, by a process using the steam reforming method for generating hydrogen from a hydrocarbon such as liquefied petroleum gas or liquefied natural gas (including a desulphurization process, a steam reforming process, and a hydrogen purification (PSA system) process). However, since hydrogen used as a carrier gas is required to have an extremely high purity as well as ammonia as a source gas, highly-pure hydrogen obtained, for example, by permeating through palladium alloy membrane is used as a carrier gas.
The ammonia used in the production process as described above is highly priced and purified. However, the acceptable concentration of ammonia contained in the discharge gas is 25 ppm because of the high toxicity. Thus, discharge gas containing such ammonia emitted into the atmosphere has harmful effects on the human body and the environment. Accordingly, it is necessary to first conduct the decomposition or the removal, or the like of ammonia contained in discharge gas discharged from the production process and to then emit the purified gas into the atmosphere. For example, Patent documents 1 to 3 disclose a processing method including bringing discharge gas containing ammonia into contact with an ammonia decomposition catalyst on heating to decompose the ammonia into nitrogen and hydrogen.
Moreover, Patent document 4 proposes a method of recovering ammonia gas having a dissolution step of dissolving ammonia of discharge gas discharged from the processing step of the production process in water, a distillation process of distilling the aqueous ammonia in which ammonia is dissolved to separate ammonia from water, and a liquefaction step of liquefying the separated ammonia. In addition, Patent document 5 proposes a method of recovering ammonia including aerating discharge gas containing ammonia discharged from the production process of a gallium nitride compound semiconductor and the like in a multitubular adsorber filled with ammonia adsorbent on cooling to adsorb and collect ammonia from the discharge gas; and removing ammonia under reduced pressure while heating the multitubular adsorber.